IOSR Journal of Mechanical and Civil Engineering (IOSR-JMCE)
e-ISSN: 2278-1684,p-ISSN: 2320-334X, Volume 14, Issue 1 Ver. I (Jan. - Feb. 2017), PP 38-46
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Comparative Study on Dynamic Analysis of Elevated Water Tank
Frame Staging and Concrete Shaft Supported
Mor Vyankatesh K. 1, More Varsha T. 2
1
(PG Student, Department of civil Engineering, J.N.E.C, Aurangabad, Maharashtra, India)
(Assistant Professor, Department of civil Engineering, J.N.E.C, Aurangabad, Maharashtra, India)
2
Abstract: This paper presents comparative study of elevated water tanks subjected to dynamic loading
supported on RC framed structure and concrete shaft structure with different capacities and placed in different
seismic zones. History of earthquake reveals that it have caused numerous losses to the life of people in its
active time, and also post earthquake time have let people suffer due to damages caused to the public utility
services. Either in urban or rural areas elevated water tanks forms integral part of water supply scheme, so its
functionality pre and post earthquake remains equally important. These events showed that importance of
supporting system is uncompromising for elevated tank as compared to any other type of tank. Damages caused
are the results of unsuitable design of supporting system; wrong selection of supporting system, etc. These
structures have heavy mass concentrated at the top of slender supporting system hence these structures are
especially vulnerable to horizontal forces due to earthquakes. This paper presents the dynamic analysis of
elevated water tanks with respect to the latest IS code published for liquid retaining structures by Bureau of
Indian Standards i.e. IS 1893 (Part 2) : 2014. Comparison of elevated tanks with different supporting system,
capacities and seismic zones states that these parameters may considerably change the seismic behaviour of
tanks.
Keywords: RC framed supported, concrete shaft supported, Convective hydrodynamic pressure, Impulsive
hydrodynamic pressure, STAAD Pro V8i.
I.
Introduction
1.1 General
Indian sub-continent is highly vulnerable to natural disasters like earthquake, draughts, floods, cyclones
etc. According to IS code 1893(Part 1):2000, more than 60% of India is prone to earthquakes. The earthquake of
26 January 2010 in Gujarat was unprecedented for the entire country, then public learnt first time that the scale
of disaster could have been far lower had the construction in the region compiled with codes of practice for
earthquake prone regions. These natural calamities are causing many casualties and innumerable property loss
every year. After an earthquake the loss which cannot be recovered are the life loss. Collapse of structures
causes people to life loss. Hence badly constructed structures kill people more than earthquake itself. Hence it
becomes important to analyse the structures properly.
Seismic safety of liquid storage tanks is of considerable importance, as tanks storing highly
concentrated liquids in industries, or in transporting vehicles, can cause considerable harm for human society if
damaged. Water supply being the lifeline facility must remain functional following disaster to carter the need of
drinking and fire fighting. Since elevated water tanks are integral part of water supply scheme, its study and
improvising it becomes need of time. Elevated water tanks can be classified depending upon its shape and
supporting.
Based on shape, elevated tanks can be classified as;
1. Circular Tank.
2. Rectangle Tank.
3. Square Tank.
4. Conical Tank.
5. Intze Tank.
Based on supporting system, elevated tanks can be classified as;
1. Shaft supported elevated tanks.
2. Framed staging / Trestle supported elevated tanks.
Damages to the elevated tanks with both type of supporting system stated above are noted during
earthquake. Hence it becomes necessary to evaluate the optimum selection of supporting system for the
predefined requirements of tank. This paper presents comparison of elevated tanks with different supporting
system, capacities, and seismic zones. All the seismic analysis parameters are evaluated using the recommended
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Comparative Study On Dynamic Analysis Of Elevated Water Tank Frame Staging And Concrete...
procedure in latest code as well as in IIT-GSDMA guidelines, and is concentrated mainly to the Sloshing effect
that is happening in the water during earthquake.
1.2 Literature Review:
A Much of literature has been presented in the form of technical papers till date on dynamic analysis of
elevated water tanks RC framed supported and concrete shaft supported. Different points are covered in that
relevance i.e. dynamic analysis, sloshing effect on tank, dynamic response of framed staging etc. Some of them
are listed below.
George W. Housner [1]: Chilean earthquake that took place in 1960 was the main plot behind this
paper. He stated about the relation between motion of water w.r.t tank and whole structure w.r.t ground. Fully
filled tank, empty tank, partially filled tank, were the three cases considered by him. Sloshing effect was
neglected in first two cases as there is no free board in first case and other no water to cause sloshing motion.
Here the whole structure behaves as one-mass structure. But in the third case sloshing effect must be considered,
because here the structure behaves as two-mass structure. Concluding he stated that the maximum force to
which a half-filled tank can be subjected is less than that of totally filled tank.
Dr. Suchita Hirde & Manoj Hedaoo [2]: Hydrodynamic analysis of elevated water tanks for various
heights, capacity and soil conditions. The effect of height of water tank, earthquake zones and soil conditions on
earthquake forces have been presented in this paper. They considered RCC circular tank with M-20 grade of
concrete and Fe-415 grade of steel for analysis. Capacity of 50,000 lit and 100.000 with staging height of 12m,
16m, 20m, 28m with 4m height of panel are considered for analysis. Following were the conclusions made in
the paper [1] Seismic forces are directly proportional to the seismic zones. [2] Seismic forces are inversely
proportional to the height of the supporting system. [3] Seismic forces increase with increase in capacity of tank.
[4] Seismic forces are higher in soft soil than that of medium and higher in medium soil than that of hard.
R Livaoglu & Dogangun [3]: This paper presents the response of the supporting system of water
towers. Here they have considered frame staging as well as concrete shaft as supporting system for elevated
water tanks. In this paper they concluded that where there is high risk of seismic force, the cylindrical shaft
support system may be used because of having important advantages than the common used frame type system.
They also found that roof displacement response for frame support is higher than that of concrete shaft support
system.
Prasad S. Barve, Ruchi P. Barve [4]: This paper states the seismic behaviour of intze tank supported on
concrete shaft. Here the study is done in accordance with the change in aspect ratio i.e. h/d ratio where, h is
height of water in tank and d is internal diameter of tank. It concludes that with increase in h/d ratio, the
impulsive fluid pressure increases with increase in base shear and base moment for that tank. As h/d ratio
increases there is decrease in convective fluid pressure of the tank, since convective mass of water comprises of
sloshing effect observed during earthquake hence it’s necessary to pick optimum h/d ratio while designing tank
components.
1.3 Objective
[1] The main objective of this paper is to study the hydrodynamic effect on elevated water tank, with different
supporting system i.e. framed staging and concrete shaft placed in different seismic zones.
[2] Also to compare the analysis results of base shear and base moment with different capacities. Comparing
impulsive and convective pressure results as they may exert pressure in different magnitude.
II.
System Development
2.1 Impulsive and Convective Mass
During an earthquake, elevated water tank with free liquid surface is subjected to horizontal ground
motion, and liquid in tank as well as tank wall are subjected to horizontal acceleration. Here the liquid in the
lower region of the tank behaves like a mass that is rigidly connected to tank wall. This mass is stated as
impulsive liquid mass (mi), which accelerates along with the wall and induces impulsive hydrodynamic pressure
on tank wall as well as base. Liquid mass in the upper region o4n the tank undergoes sloshing motion. This mass
is stated as convective liquid mass (mc) and it exerts convective hydrodynamic pressure on tank wall and base.
Thus the total liquid mass gets divided into two parts i.e. impulsive mass and convective mass. These mass are
suitably represented in spring mass model.
A qualitative description of impulsive and convective hydrodynamic pressure distribution on tank wall is given
in Figure 1
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2.2 Spring Mass Model for Seismic Analysis of Elevated Tank
Most of the elevated tanks are partially filled. Hence two-mass idealization of the tank is more
appropriate than one-mass model. It is also being commonly used in international codes. The response of twodegree of freedom system can be obtained by elementary structural dynamics. However, two periods are well
separated for most of the tanks. Hence, system can be considered as two uncoupled single degree of freedom
system. This two uncoupled single degree of freedom systems, one representing impulsive plus structural mass
behaving as an inverted pendulum with lateral stiffness equal to the stiffness of staging, (ks) and the other
convective mass with spring of stiffness, (kc).
Fig.2 (a) Elevated tank
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Fig.2 (b) Spring mass model
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Comparative Study On Dynamic Analysis Of Elevated Water Tank Frame Staging And Concrete...
Fig.2 (c) Two-mass idealization of elevated tank
Fig.2 (d) Equivalent uncoupled system
2.3 Lateral Stiffness
Lateral stiffness of staging is defined as the force required to be applied at the CG of tank so as to get a
corresponding unit deflection. From the deflection of CG of tank due to an arbitrary lateral force one can get
stiffness of frame staging. STAAD pro software is used to model the staging.
Fig.3 Deflected shape of tank staging for an arbitary force
Lateral stifness of concrete shaft is calculated according to the guidelines by IITK-GSDMA. Here,
shaft is considered as cantilever about the height of shaft from top of footing upto bottom of circular ring beam.
Lateral stiffness is given by: Ks = 3 EI/L3 ..(1). Where, E = Modulus of elasticity, I = Moment of inertia of shaft
cross section, L = Height of shaft.
Minimum thickness of concrete shaft:
As per IS 1893 (Part 2): 2014, clause 8.2.1 minimum thickness of shaft is recommended as 150mm for
shaft diameter upto 4m. Shaft with diameter leass than 8m are calculated by: t min = 150 + (D-4000)/80 ..(2) , and
for shafts with diameter equal to or greater than 8m are claculated by : t min = 200 + (D-8000)/120 ..(3) where, D
= diameter of shaft in mm.
2.4 Numerical Statement
Dynamic analysis of circular elevated water tanks, supported on RC frame staging and concrete shaft
structure of different capacities and following specifications are performed as per IS 1893 (Part 2): 2014 for
medium soil condition and zone II, III, IV, V. As capacity increases, number of columns supporting in frame
staging structure of tank are increased with sizes of components kept constant for all tanks. Where as for tanks
supported on concrete shaft, the sizes of components are changed as shown below with increase in capacity of
tank. A 1.2m wide gallery is considered around the periphery of all tanks for access. Grade of concrete, grade of
steel and soil condition for circular elevated tanks are M-20, Fe415 and medium soil respectively. All tanks are
provided with appropriate free board.Here, H = Lowest supply level from ground, D= Internal diameter of tank,
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ds= Diameter of shaft, h= Height of water in tank from bottom of wall, N= Number of columns, t= Thickness of
shaft, where as, the ratio of H/D and h/D is kept constant for all capacities of tank.
Table 1: Geometrical Specifications for frame supported tanks.
Tank
T1
T2
T3
T4
Capacity
(m3)
150
250
500
750
H
(m)
12
14
18
21
D
(m)
8
9.5
12
14
h
(m)
3
3.5
4.5
5
H/D
-
h/d
-
1.5
(±0.1)
0.36
(±0.1)
N
(no’s)
5
9
13
17
Table 2: Geometrical Specifications for shaft supported tanks.
Tank
S1
S2
S3
S4
Capacity
(m3)
150
250
500
750
H
(m)
12
14.4
18
21
D
(m)
7.8
8.6
11.2
13
ds
(m)
5.5
6.13
8
8.9
h
(m)
3
4
4.5
4.9
H/D
-
h/d
-
1.5
(±0.1)
0.36
(±0.1)
t
(mm)
150
180
200
220
Please note the ratio h/d for tanks supported on concrete shaft are calculated from equivalent circular tank of
same volume and diameter equal to diameter of tank at top level.
Table 3: Constants.
Sr No.
1
2
3
Constant
Seismic intensity (Zone II)
Zone III
Zone IV
Zone V
Response reduction factor (R), for tanks supported on frame staging
Response reduction factor (R), for tanks supported on concrete shaft
Importance factor (I)
Values
0.1 (as per IS code 1893 Part 2)
0.16
0.24
0.36
4 (as per IS code 1893 Part 2)
3.5 (as per IS code 1893 Part 2)
1.5
Table 4: Components Sizes for frame supported tanks.
Sr No.
1
2
3
4
5
6
7
8
Component
Roof slab
Cylindrical wall
Base slab
Roof beams
Floor beams
Braces
Gallery
Columns
Sizes (mm)
150
200
200
230 x 450
300 x 600
300 x 450
110
500
Table 5: Components Sizes for shaft supported tanks
(Sizes changes as per increase in tank capacity)
Sr No.
1
2
3
4
5
6
7
Component
Top Dome
Cylindrical wall
Top ring beam
Bottom ring beam
Circular ring beam
Bottom Dome
Conical Dome
Sizes (mm)
110 to 150
180 to 220
250x300 to 300x450
300x500 to 300x600
500x600 to 500x750
200 to 220
230 to 250
III. Results And Discussion
For all the above stated Circular elevated tanks, RC frame staging system was modelled in STAAD Pro
and arbitrary load was applied at the centre of gravity of the tank. Following are the values of stiffness
calculated (T1 toT2). Also stiffness for tanks supported on concrete shaft is calculated below (S1 to S2).
Table 6: Stiffness.
Tank
T1
T2
T3
T4
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Stiffness (Kn/m)
10570.82
12156.58
17241.38
18867.92
Tank
S1
S2
S3
S4
Stiffness (Kn/m)
259911
270301
363334
413102
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3.1 Time Period
Time period in impulsive mode (Ti) and time period in convective mode of vibration (T c) is calculated
and compared with capacities of tank. Since time period is same for all seismic zones, only single curve
represents time period in all zones below.
Fig.4 Capacity v/s Time period (impulsive mode)
Fig.5 Capacity v/s Time period (convective mode)
3.2 Base Shear
Total base shear (V), the horizontal force which acts at the bottom at the staging is resultant of two
different case base shears, one for impulsive mode (V i) and for convective mode (Vc). It represents the increase
in base shear with increase in capacity of tank.
Fig.6 Capacity v/s Total base shear (Zone II)
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Fig.7 Capacity v/s Total base shear (Zone III)
Fig.8 Capacity v/s Total base shear (Zone IV)
Fig.9 Capacity v/s Total base shear (Zone V)
3.3 Base Moment
Since the large mass accumulation at the top of slender supporting system, the overturning moment is
the important parameter at the time of designing elevated water tank. Its observed that base moment for tanks
supported on frame staging is less than that of tanks supported on concrete shaft.
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Fig.10 Capacity v/s Total Base Moment (Zone II)
Fig.11 Capacity v/s Total Base Moment (Zone III)
Fig.12 Capacity v/s Total Base Moment (Zone IV)
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Fig.13 Capacity v/s Total Base Moment (Zone V)
3.4 Sloshing Wave Height
Free Board to be provided in tank based on the maximum value of sloshing wave height. This is
particularly for the important tanks containing toxic liquids, where loss of liquid needs to be prevented. Or if
required free board is not provided, roof structure of tank should be deigned for resisting uplift pressure due to
sloshing liquid.
Table 7: Sloshing Wave Height.
Tank
T1
T2
T3
T4
S1
S2
S3
S4
Zone II
129
140
158
178
130
139
157
169
Sloshing wave height (mm)
Zone III
Zone IV
207
310
225
337
253
380
285
428
208
313
223
335
252
379
270
405
Zone V
466
506
570
642
470
503
568
608
IV. Conclusion
Following conclusions are made based on the aforementioned analysis presented in this paper.
1) Base shear for elevated tanks, supported on concrete shaft is greater than that of elevated tanks supported on
frame staging.
2) Base moment being important parameter while designing the structure is considerably greater for tanks with
concrete shaft supported. Hence the areas with high seismic intensity, threat to tank with shaft supporting
are more than that of staging support.
3) Time period in impulsive mode for shaft supported tanks and frame supported tanks differ subsequently.
But for convective mode the difference is less comparatively.
4) The deflection of staging is found to be decreasing with increase of capacity and change in staging pattern,
further causing increase in its stiffness.
5) Sloshing wave height is approximately same for the tanks with different supporting system, but it differs for
tanks as the capacity increases.
References
[1].
[2].
[3].
[4].
[5].
[6].
[7].
[8].
[9].
[10].
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Dr Suchita Hirde, Dr Manoj Hedaoo, “Seismic Performance of Elevated Water Tank”, International Journal of Advanced
Engineering Research and Studies/ Vol.I/2011.
Prasad S Barve, Ruchi P Barve 2015 “Parametric Study to understand the Seismic Behaviour of Inze Tank Supported on Shaft”
International journal of engineering sciences & research technology Barve, 4(7): July, 2015
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Sudhir K Jain and M. S. Mdhekar, October-1993, “Proposed provisions for aseismic design of liquid storage tanks” Journals of
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